This invention relates to a microwave heating system used as a heating magnetron and, more particularly, to such a system having an arrangement to stabilize the output power of the magnetron against fluctuations which might be caused by variations in line voltage of an AC input line.
Microwave ovens commonly use 60 Hz LC power supply systems. Such power supply systems energize cooking magnetrons and exhibit very good regulation of magnetron current relative to line voltage. That is, the magnetron current (and therefore the magnetron power) stays relatively stable independent of variations in the line voltage of the AC input line powering the microwave oven. Such power supply systems have a transformer which allows the secondary to saturate the core by means of the positioning of a magnetic shunt within the main core. The saturation of the secondary maintains its voltage substantially constant. As the relationship between the magnetron plate voltage and plate current is generally linear for a magnetron which is operating to generate microwave power, the constant voltage at the secondary maintains a constant magnetron current and, therefore, the magnetron generates a constant power. This is of course true only if the working impedance of the magnetron load remains constant.
The ability of LC power supply systems to provide this stabilization of magnetron power against fluctuations due to variations in line voltage without complex additional circuits is one of many reasons that such power supply systems have been almost universally used in commercially available microwave ovens. However, such LC power supply systems have had disadvantages such as the need for a relatively large, heavy, and expensive power transformer.
One problem which has hindered the implementation of alternatives to the LC power supply systems is the need to regulate magnetron current against fluctuations due to line voltage variations. Alternates to the LC power supply system may not allow the use of the same mechanism to stabilize magnetron current and magnetron power. For example, one might use a high frequency inverter for powering a microwave oven, a relatively low cost transformer employing two "E" cores and one main winding assembly of secondary wound tightly on the primary. However, this would not usually permit the placement of a magnetic shunt or allow the secondary to saturate the core without causing saturation of the primary. Saturation of the primary is of course unacceptable. Further, the additional cost of adding a magnetic shunt to a high frequency core and associated increased size, additional weight, and great difficulty in maintaining leakage value tolerance in production all are serious obstacles to the regulation of magnetron current by saturation of a secondary in a high frequency inverter power supply.
Accordingly, it is a primary object of the present invention to provide a new and improved technique of magnetron power regulation.
A more specific object of the present invention is to provide an inverter driven magnetron power supply with efficient stabilization of the magnetron power output.